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Hydrocephalus is defined as increased volume of cerebrospinal fluid within the cerebral ventricles. This is manifested by dilation of some or all of the cerebral ventricles, most often the lateral ventricles. Hydrocephalus may result from a variety of etiologies, including genetic syndromes, congenital anomalies of the brain and spinal cord, and in utero infection or exposure to teratogens. Hydrocephalus is commonly associated with other fetal abnormalities, often involving intracranial structures or the spine. The prognosis for a fetus with hydrocephalus is related to the degree and severity of associated abnormalities and to the cerebral cortical thickness.

Hydrocephalus is diagnosed sonographically when there is abnormal dilation of the cerebral ventricles. From 18 weeks gestation onward, criteria for hydrocephalus include width of the lateral ventricle at the atrium measuring more than 10 mm and choroid plexus dangling from its medial attachment toward the lateral wall of the lateral ventricle. Before 18 weeks, the diagnosis of hydrocephalus is based on the appearance of the lateral ventricle and dangling choroid plexus because ventricular dilation may be present with a ventricular width less than 10 mm . When hydrocephalus is diagnosed based on dilation of the lateral ventricles, it is important to assess the third and fourth ventricles for dilation. A dilated third ventricle is diagnosed when there is abnormal separation of the walls of the third ventricle by cerebrospinal fluid . Increased fluid in the fourth ventricle is best seen on axial or sagittal views of the posterior fossa. Because hydrocephalus is commonly associated with other anomalies of the central nervous system, the fetal cranium and posterior fossa should be evaluated. The cranium should be assessed, looking for a lemon shape that would suggest a meningomyelocele or an opening with herniated tissue that would indicate an encephalocele. The posterior fossa should be assessed for evidence of a Dandy-Walker malformation or the Chiari II malformation associated with meningomyelocele. If the posterior fossa is normal, the diagnosis of aqueductal stenosis should be considered.

Hydrocephalus A: Axial view of a fetal head at 35 weeks gestation demonstrates a dilated lateral ventricle with + calipers measuring the width of the atrium of lateral ventricle as 18.9 mm. The calipers are aligned perpendicular to the axis of the ventricle. B: A 30-week fetus with dilated ventricles (+ calipers). Choroid plexus (arrowhead) is seen outlined by fluid, extending from its medial wall attachment to the lateral wall of the ventricle.

Hydrocephalus with a dangling choroid plexus Axial view of an 18-week fetus with hydrocephalus demonstrates the choroid plexus (arrowheads) dangling from its medial attachment toward the lateral wall of the ventricle.

Hydrocephalus in a 16-week fetus Severely dilated ventricle (+ calipers) with dangling choroid plexus (arrowhead). Measurement is less than 10 mm (9.8 mm) despite the presence of hydrocephalus, because of the early gestational age.

Dilated third ventricle with hydrocephalus Axial view of the head at the level of the thalami and third ventricle (arrows) demonstrates dilation in this fetus with hydrocephalus from congenital toxoplasmosis.

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